The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with imaging body tissue in selected planar regions with primary-stimulated imaging sequences and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also applicable to imaging and analysing of selected regions of animate and inanimate objects with field echo, spin echo, and other imaging sequences.
Heretofore, various imaging sequences have been utilized in medical diagonstic and other magnetic resonance imaging, e.g. spin echo sequences. A common imaging sequence included applying a 90.degree. radio frequency excitation pulse to induce magnetic resonance in selected dipoles in an image region. A phase encoding gradient pulse was applied to encode the resonating dipoles with a selected phase angle. A magnetization rotation pulse, commonly a 180.degree. inversion pulse, induced magnetization vectors of the resonating dipoles to converge toward a spin echo. Imaging data was then collected during the induced echo. To generate a 256 view image, the sequence was repeated 256 times, each time with a different phase angle encoding.
For some examinations, it was advantageous to generate images which were dominated by data attributable to the longer T1 relaxation times. In one technique for generating T1 rather than T2 weighted images, a 90.degree. excitation pulse was followed by two 90.degree. magnetization rotation pulses. A phase encode gradient was applied either between the excitation pulse and the second 90.degree. pulse or between the third 90.degree. pulse and an induced stimulated echo. The phase encode gradient caused the magnetic resonance data collected during the stimulated echo to have an appropriate phase angle encoding. To generate an image with 256 views, the sequence was repeated 256 times, each time with a different phase angle encoding gradient. Optionally, additional magnetization vector rotation pulses of an appropriate rotational angle were applied to cause additional stimulated echoes in the same sequence. By applying an additional phase encode gradient pulse after each magnetization vector rotation pulse, the phase encoding of the successive stimulated echoes could be altered. See for example, "Rapid Images and NMR Movies" A. Haase, et al., SMRM Abstracts, pages 980-981 (1985).
Various techniques were also developed for accelerating the data collection time for T2 weighted images. Typically information regarding more than one view was acquired during the sequence following each excitation. As one example, this was achieved by utilizing a Carr-Purcell sequence in which an appropriate phase encode gradient was applied adjacent each inversion pulse to alter the phase encoding of the following spin echo. For example, a 90.degree. magnetic resonance excitation pulse was applied to excite magnetic resonance. A phase encoding gradient was applied followed by a 180.degree. inversion pulse to induce a first spin echo. After collecting data during the first spin echo, another phase encoding pulse was applied to alter the phase encoding and a second 180.degree. inversion pulse was applied to cause a second spin echo with the different phase encoding. The changing of the phase encoding and the application of an additional 180.degree. inversion pulse was repeated a plurality of times following a single, initial excitation. See for example, "Phase-Encoded, Rapid Multiple-Echo (PERME) Imaging" M. Lawton, et al., SMRM Abstracts, pages 1009-1010 (1985).
One of the drawbacks to the multiple echo Carr-Purcell techniques was that the signal intensity varied greatly from echo to echo. Commonly, the T2 relaxation times in the body are short compared to the interval between inversion pulses in the multiple echo Carr-Purcell sequence. This variation caused the signal intensity to vary significantly from echo to echo, which greatly degraded image quality.
The present invention provides a new and improved magnetic resonance sequence which provides a unique and selectable weighting between T1 and T2 to overcome the above referenced problems and others.